1. Field of the Invention
The present invention is related to thin-film solid-state batteries and, in particular, the deposition of LiCoO2 films and layers for battery manufacture.
2. Discussion of Related Art
Solid-state thin-film batteries are typically formed by stacking thin films on a substrate in such a way that the films cooperate to generate a voltage. The thin films typically include current collectors, a cathode, an anode, and an electrolyte. The thin films can be deposited utilizing a number of deposition processes, including sputtering and electroplating. Substrates suitable for this application have conventionally been high temperature materials capable of withstanding at least one high temperature anneal process to at least 700° C. for up to about 2 hours in air so as to crystallize the LiCoO2 film. Such a substrate can be any suitable material with appropriate structural and material properties, for example a semiconductor wafer, metallic sheet (e.g., titanium or zirconium), ceramic such as alumina, or other material capable of withstanding subsequent high temperature processing in the presence of the LiCoO2, which can experience significant interfacial reactions with most materials utilized in a battery during these temperature cycles.
Other lithium containing mixed metal oxides besides LiCoO2, including Ni, Nb, Mn, V, and sometimes also Co, but including other transition metal oxides, have been evaluated as crystalline energy storage cathode materials. Typically, the cathode material is deposited in amorphous form and then the material is heated in an anneal process to form the crystalline material. In LiCoO2, for example, an anneal at or above 700° C. transforms the deposited amorphous film to a crystalline form. Such a high temperature anneal, however, severely limits the materials that can be utilized as the substrate, induces destructive reaction with the lithium containing cathode material and often requires the use of expensive noble metals such as gold. Such high thermal budget processes (i.e., high temperatures for extended periods of time) are incompatible with semiconductor or MEM device processing and limit the choice of substrate materials, increase the cost, and decrease the yield of such batteries.
It is known that crystallization of amorphous LiCoO2 on precious metals can be achieved. An example of this crystallization is discussed in Kim et al., where a conventional furnace anneal at 700° C. for 20 minutes of an amorphous layer of LiCoO2 on a precious metal achieves crystallization of the LiCoO2 material, as shown by x-ray diffraction data. Kim, Han-Ki and Yoon, Young Soo, “Characteristics of rapid-thermal-annealed LiCoO2, cathode film for an all-solid-state thin film microbattery,” J. Vac. Sci. Techn. A 22(4), July/August 2004. In Kim et al., the LiCoO2 film was deposited on a platinum film that was deposited on a high-temperature MgO/Si substrate. In Kim et al, it was shown that such a crystalline film is capable of constituting the Li+ ion containing cathode layer of a functional all solid-state Li+ ion battery.
There are many references that disclose an ion beam assisted process that can provide a LiCoO2 film that demonstrates some observable crystalline composition by low angle x-ray diffraction (XRD). Some examples of these are found in U.S. patent application Ser. Nos. 09/815,983 (Publication No. US 2002/001747), 09/815,621 (Publication No. US 2001/0032666), and 09/815,919 (Publication No. US 2002/0001746). These references disclose the use of a second front side ion beam or other ion source side-by-side with a deposition source so as to obtain a region of overlap of the flux of ions with the flux of LiCoO2 vapor at the substrate surface. None of these references disclose film temperature data or other temperature data of the film during deposition to support an assertion of low temperature processing.
It is very difficult to form a uniform deposition either by sputtering a material layer or by bombardment with an ion flux. Utilization of two uniform simultaneous distributions from two sources that do not occupy the same position and extent with respect to the substrate enormously increases the difficulties involved in achieving a uniform material deposition. These references do not disclose a uniform materials deposition, which is required for reliable production of thin-film batteries. A well understood specification for material uniformity for useful battery products is that a 5% one-sigma material uniformity is standard in thin film manufacturing. About 86% of the films with this uniformity will be found acceptable for battery production.
It is even more difficult to scale a substrate to manufacturing scale, such as 200 mm or 300 mm. Indeed, in the references discussed above that utilize both a sputtering deposition and an ion beam deposition, only small area targets and small area substrates are disclosed. These references disclose a single feasibility result. No method for achieving a uniform distribution from two separate front side sources has been disclosed in these references.
Further, conventional materials and production processes can limit the energy density capacity of the batteries produced, causing a need for more batteries occupying more volume. It is specifically desirable to produce batteries that have large amounts of stored energy per unit volume in order to provide batteries of low weight and low volume.
Therefore, there is a need for a low temperature process for depositing crystalline material, for example LiCoO2 material, onto a substrate. In particular, there is a need for processes that allow production of cathodic lithium films for a battery structure with a low enough thermal budget to allow production of functional structures on low temperature materials such as stainless steel, aluminum, or copper foil.